Hydro-mechanical coupling effect on surficial layer stability of unsaturated expansive soil slopes

Rain-induced surficial slope failures in expansive soils are frequently reported in the literature. These failures are primarily attributed to the shear strength reduction due to the loss of suction associated with the water infiltration. For this reason, slope stability is conventionally evaluated taking account of the hydraulic response of the soil to infiltration of water. The unsaturated expansive soil swells significantly upon wetting; the associated hydro-mechanical coupling effect on the slope stability of expansive soils is not considered in conventional geotechnical engineering practice. This paper aims to numerically evaluate the coupling effect or swelling on the hydraulic response as well as the stability of surficial layer of a typical expansive soil slope. Both hydro-mechanical (coupled) and hydraulic (uncoupled) responses of the slope to a low intensity prolonged rainfall are modeled using the commercial software SIGMA/W and SEEP/W, respectively. Subsequently, the infinite slope formulation is used to compute the factors of safety (FS) profiles extending both coupled (SIGMA/W) and uncoupled (SEEP/W) analysis. The results of the study presented in this paper show that coupled analysis (considering swelling) leads to different suction (negative pore water pressure (PWP)) and FS profiles within the surficial layer from those resulting from uncoupled analysis at the same elapsed time. The wetting fronts in the PWP profile from coupled analysis advance at a relatively faster rate in comparison to uncoupled analysis contributing to more critical FS values or failure conditions. The study highlights the coupled hydro-mechanical behavior of expansive soil has an adverse effect on the slope stability and hence should be taken into account in practice for estimating reliable values of FS.